1. Field of the Invention
The present invention relates to an audio signal recording system and, more particularly, to a system of this kind that records on a recording medium a stereophonic audio signal as a sum signal and a difference signal of left and right signals thereof.
2. Description of the Related Art
A system which records a stereophonic audio signal as a signal expressing the sum (L+R) of a left (L) signal and a right (R) signal and a signal expressing the difference (L-R) therebetween is suitable for use in a system in which one of two recording channels is mainly used and the other is used supplementarily.
This is because a monophonic audio signal can be reproduced even when no signal has been recorded in the supplementarily used recording channel.
FIG. 1 shows an example of the arrangement of such a recording apparatus. The recording apparatus has terminals 20 and 21 to which signals L and R are respectively input, a matrix circuit 22 for forming signals (L+R) and (L-R) from the signals L and R, modulators 23 and 24 for modulating the signals (L+R) and (L-R), respectively, and a recording and reproducing section 25 capable of simultaneously recording or reproducing signals for two channels which have been output by the modulators 23 and 24.
Signals for two channels which have been reproduced by the recording and reproducing section 25 are supplied to demodulators 26 and 27 wherein signals (L+R) and (L-R) for the basebands are obtained respectively. These signals output from the demodulators 26 and 27 are supplied to a matrix circuit 28 whereby signals L and R are extracted. The signals are then output from terminals 29 and 30.
One example of a system in which a stereophonic audio signal is transmitted as a sum (L+R) signal and a difference (L-R) signal of left and right signals L and R thereof is the so-called television sound multiplex broadcast system.
FIGS. 2(A) and 2(B) show the spectral distributions of signals transmitted by a television sound multiplex broadcast system. FIG. 2(A) shows the spectral distribution of a television audio multiplex signal in one channel, in which the symbol Y represents the spectral distribution of a luminance signal, C represents that of a carrier chrominance signal, and A represents that of an audio multiplex signal. Further, the symbol VC represents a carrier of the video signal, AC represents an FM carrier of the audio signal, and CSC represents a color subcarrier.
FIG. 2(B) shows the spectral distributions of signals transmitted by the FM carrier AC of the audio signal, in which the symbols A1 and A2 represent bands occupied by a main audio signal and a sub-audio signal, respectively, ASC represents an FM subcarrier of the sub-audio signal, and X represents a control signal for discrimination. The control signal X for discrimination does not exist when the main audio signal and the sub-audio signal are signals (L+R) and (L-R), whereas, it does exist when the main audio signal and the sub-audio signal are first and second audio signals, respectively; the first audio signal is a main audio signal, and the second audio signal is an audio signal different from the first audio signal, for instance, a foreign language signal in bilingual broadcasting.
FIG. 3 shows a typical arrangement of a known apparatus for receiving a television audio multiplex signal such as that shown in FIGS. 2(A) and 2(B). Reference numeral 1 denotes a tuner unit which outputs a television audio multiplex signal, such as that shown in FIG. 2(A), being transmitted over one channel.
The apparatus also has a video detecting circuit 2 which performs the AM detection of a carrier VC (in FIG. 2(A)) of a video signal and generates a video signal which is then output through a video signal output terminal 3.
Reference numeral 4 denotes an audio FM detecting circuit which performs the FM detection of an FM carrier AC (in FIG. 2(A)) of an audio signal and generates an audio multiplex signal such as that shown in FIG. 2(B). A low-pass filter (LPF) 5 is provided for the purpose of extracting a main audio signal from the audio multiplex signal, and a band-pass filter (BPF) 6 is provided for the purpose of extracting a frequency-modulated sub-audio signal from the audio multiplex signal. An FM detecting circuit 7 performs the FM detection of the frequency-modulated sub-audio signal which has thus been extracted by the BPF 6, and then the circuit 7 outputs a detected sub-audio signal.
Another BPF 8 separates a discrimination control signal X from the audio multiplex signal, and the thus separated discrimination control signal X is supplied to an AM detecting circuit 9. The output of the circuit 9 is at a high level when the discrimination control signal X exists, whereas, it is at a low level when the signal X does not exist. Switches 10 and 11 are changeover switches each of which is connected to a side B when the output from the AM detecting circuit 9 is high and is connected to a side A when that output is low. Reference numeral 12 denotes a matrix circuit which comprises an adder and a subtracter and which outputs signals L and R. Accordingly, when the discrimination control signal X does not exist, audio output terminals 13 and 14 output the signals L and R, whereas, when the control signal X exists, they output first and second audio signals.
The so-called M-S microphones are known as microphones employed mainly in commercial use. An M-S microphone comprises a first unidirectional microphone unit directed to the front, and a second bidirectional microphone unit directed to the sides, which units are accommodated in a single housing. A signal M output from the first microphone unit and a signal S output from the second microphone unit are synthesized by a summation-difference matrix circuit (not shown), thereby obtaining signals L and R.
FIG. 4(A) shows the directivities (m) and (s) of the first and second microphone units of a typical M-S microphone. FIG. 4(B) shows an example of the directivities (l) and (r) of signals L and R which have been obtained by synthesizing the signals S and M in the summation-difference matrix circuit.
With an M-S microphone such as that illustrated, if the magnitude of the output from the second microphone unit is varied, the angle 8 at which the stereophonic sound spreads can be changed. The M-S microphone is advantageous in that it enables the spreading angle .theta. to be electrically adjusted by remote control.
When an audio signal is output to a sound outputting device such as speakers, the audio signal must be in the form of L and R signals. However, in the case where the output of the receiving apparatus such as that shown in FIG. 3 or the output of the M-S microphone such as that described above is to be recorded by the recording apparatus such as that shown in FIG. 1, audio signals (L+R) and (L-R) for two channels are first converted into signals L and R by being passed through a matrix circuit provided for the microphone or the receiving apparatus, and are then passed through another matrix circuit 22 before they are recorded. If the signals are passed through matrix circuits twice in this way, there is a risk of the left-and-right separation characteristic of a stereophonic audio signal being deteriorated. Further, noise components may tend to become mixed. In such cases, therefore, it is impossible to perform good recording of the stereophonic audio signal.